2Q2 METALLIC COATINGS METALLIC COATINGS Many metallic objects are coated, either to render them more resistant to the action of external agencies or to enhance their appearance, with more or less thin layers of other metals or of oxides. These coatings are easy to characterise by the tests given below. GOLD-PLATING (Gilding) 1. Technical Test. — The surface of the object is rubbed repeatedly with a small piece of very fine glass paper (No. ooo) so as to concentrate any gilding at one point of the glass paper. This point is then treated with a drop of cone, nitric and one or two drops of cone, hydrochloric acid and warmed gently over a very small flame until the metal is dissolved. The solution is then washed into a test-tube with 1-2 c.c. of water, the liquid being filtered if turbid and heated with either an equal volume of fresh sulphur dioxide solution or a few drops of fresh stannous chloride solution. In presence of gold, a violet red, varying coloration is observed owing to the formation of purple of Cassius 2. Test for Small Objects. — The sample, or part of it, or a number of small pieces, according to circumstances, are heated on the water-bath with nitric acid diluted with an equal volume of water. When the attack of the common metal is complete, the liquid is filtered through a small filter and the residue washed thoroughly with hot water. The filter is then incinerated in a porcelain dish or crucible and the ash treated with 2 drops of cone. hydrochloric acid and i drop of cone, nitric acid and evaporated to dryness on a water-bath until the excess of acid is entirely expelled. When cold, the residue is taken Up in about 2 c.c. of distilled water and filtered, the filtrate being heated to boiling with an equal volume of sulphur dioxide solution or of a saturated oxalic acid solution or a few drops of stannous chloride solution. If gold is present, the characteristic violet- red coloration of purple of Cassius is observed. 3. Test for Large Objects. — After removal of any organic matter, the surface is scraped with a penknife and the scrapings submitted to the preceding test. SILVER-PLATING 1. Technical Test. — The article to be tested, freed from grease,1 is touched with a drop of cone, nitric acid and the latter absorbed by a strip of filter-paper. The spot is then treated with a drop of formaldehyde solution (commercial formalin) and a drop of 20% sodium hydroxide solution. In presence of silver, a blackish spot of reduced silver forms either immediately or after some time. 1 Sometimes objects are covered with a varnish which has nitre- or acetyl-cellulose as its base and can be removed only mechanically.f silver.